Dual-sided structured film articles

ABSTRACT

Film articles with dual-sided structures are ones in which both of the major surfaces of the film have a structured surface. The structured film articles have a first major surface and second major surface, where each surface has a plurality of spaced apart protrusions forming a repeating pattern. Each repeating pattern has a major axis, where the major axis is one of the major axes in the translational direction of the repeating pattern. The major axis of the repeating pattern on the second major surface forms an oblique angle with the major axis on the first major surface, where the angle is in the range of 10-90% of the angle of rotational symmetry of the repeating pattern. The structured film is a unitary substrate. The structured film articles are prepared by providing a flowable material composition having two major surfaces and simultaneously contacting the major surfaces with a first microstructuring tool, and a second microstructuring tool. Each microstructuring tool has a structured surface including a pattern of a plurality of depressions.

FIELD OF THE DISCLOSURE

This disclosure relates generally to film articles with dual-sidedstructures, and methods for preparing them.

BACKGROUND

A wide range of articles, such as films, sheets and the like have twomajor surfaces. It is often desirable in the fabrication of sucharticles to be able to produce a structured surface on both of the majorsurfaces. Various techniques have been developed for producing astructured surface on both major surfaces of an article.

In U.S. Pat. No. 7,165,959 (Humlicek et al.) an apparatus is describedfor casting a patterned surface on both sides of a web. The apparatusincludes two patterned rolls maintained in continuous registration towithin at least 100 microns. Similarly, U.S. Pat. No. 7,484,950(Mizunuma et al.) describes an alignment device for formingdouble-surface formed sheets. In this device a first forming roll and asecond forming roll are provided parallel and opposite to each other,and control devices are provided that reduce the phase differencebetween the forming shapes on the two surfaces.

US Patent Publication 2012/0156777 (Rangarajan et al.) describes acarrier for growing adherent cells which comprises one or more outersurfaces and one or more structured indentations on one or more of theouter surfaces.

SUMMARY

Described herein are film articles that are structured substrates withdual-sided structures, and methods for preparing them. Structuredsubstrates with dual-sided structures are ones in which both of themajor surfaces of the substrate have a structured surface, generally amicrostructured surface.

In some embodiments, the structured substrate comprises a first majorsurface and second major surface, where the first major surface and thesecond major surface each comprise a plurality of spaced apartprotrusions forming a repeating pattern. Each repeating pattern has amajor axis, where the major axis comprises one of the major axes in thetranslational direction of the repeating pattern. The major axis of therepeating pattern on the second major surface forms an oblique anglewith the major axis on the first major surface, where the angle is inthe range of 10-90% of the angle of rotational symmetry of the repeatingpattern. The structured substrate is a unitary substrate. In someembodiments, the repeating pattern on the first major surface and/or thesecond major surface comprises a periodic geometric pattern.

Also disclosed herein are methods for preparing structured articles withdual-sided structures. In some embodiments, the method of preparing anarticle comprises providing a flowable material composition having afirst major surface and second major surface, providing a firstmicrostructuring tool, the first microstructuring tool comprising astructured surface comprising a pattern comprising a plurality ofdepressions, providing a second microstructuring tool, the secondmicrostructuring tool comprising a structured surface comprising apattern comprising a plurality of depressions, and simultaneouslycontacting the first microstructuring tool to the first major surface ofthe flowable material composition and contacting the secondmicrostructuring tool to the second major surface of the flowablematerial composition to form structured first and second major surfaceson the flowable material composition. The structured first major surfaceand the structured second major surface each comprise a plurality ofspaced apart protrusions forming a repeating pattern, each repeatingpattern having a major axis. The major axis comprises one of the majoraxes in the translational direction of the repeating pattern. The majoraxis of the repeating pattern on the second major surface forms anoblique angle with the major axis on the first major surface. The angleis in the range of 10-90% of the angle of rotational symmetry of therepeating pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

The present application may be more completely understood inconsideration of the following detailed description of variousembodiments of the disclosure in connection with the accompanyingdrawings.

FIG. 1 shows a schematic of a process of this disclosure for forming adual-sided structured article.

FIG. 2 shows a schematic of a comparative process for forming adual-sided structured article.

FIG. 3 shows a schematic of a process of this disclosure for forming adual-sided structured article.

FIG. 4 shows a cross sectional view of a portion of dual-sidedstructured article of this disclosure.

FIG. 5 shows a perspective view of a comparative dual-sided structuredarticle.

FIG. 6 shows a perspective view of a dual-sided structured article ofthis disclosure.

In the following description of the illustrated embodiments, referenceis made to the accompanying drawings, in which is shown by way ofillustration, various embodiments in which the disclosure may bepracticed. It is to be understood that the embodiments may be utilizedand structural changes may be made without departing from the scope ofthe present disclosure. The figures are not necessarily to scale. Likenumbers used in the figures refer to like components. However, it willbe understood that the use of a number to refer to a component in agiven figure is not intended to limit the component in another figurelabeled with the same number.

DETAILED DESCRIPTION

It is desirable to be able to form articles that have structuredsurfaces on both major surfaces. Structured surfaces are well understoodto be ones that have a pattern imposed on them and are different fromthe natural surface roughness inherent in all surfaces. A variety ofmethods are possible and have been used to form articles with structuredsurfaces on both major surfaces. One method is to prepare two films withstructured surfaces on one side and laminate them together to form anarticle with two structured surfaces. The two films can be laminatedtogether with or without intervening layers or films, to give an articlethat has at least one interface. Such techniques can be very laborintensive, and generally require considerable care to ensure that thefilms are aligned properly prior to lamination. Additionally, thepresence of one or more interfaces provides potential weak points in thearticle, points where delamination of the article can occur. Anotherlimitation is that each of the films must be of sufficient thickness tobe handled, precluding the formation of thin articles by this technique.Another consideration is that if the materials are bonded together, abonding agent, such as an adhesive, is necessary to bond the layerstogether. Not only does this add additional expense and processingsteps, the use of, for example a pressure sensitive adhesive, cancontain low molecular weight contaminants, such as additives (e.g.tackifying agents or plasticizing agents) or unpolymerized monomers.These low molecular weight contaminants can escape from the edges of thebonded assembly. Additionally, in embodiments where optical propertiesare important, the bonding materials can adversely affect the opticalproperties.

Another technique that could be used is to form articles withstructuring on both major surfaces is to impart structuring to eachsurface separately. A flowable material such as a molten film could befirst contacted by a first structuring tool on one surface and then becontacted with a second structuring tool on the other surface at a latertime. For example, in a continuous process, an extruded film could becontacted with a structuring tool on its top surface and then becontacted by a second structuring tool on its bottom surface as the filmpasses down the line. There are a number of drawbacks to this process.Structuring at two different times and/or locations requires that thefilm be maintained in the molten state for a longer period of time. Notonly is this undesirable from a cost and energy expenditure standpoint,keeping the film molten after the first structure is imparted can causethat structure to be lost since the molten material can reflow back toits original configuration. In an alternative approach, after the firstsurface of the film is structured, the film could be cooled to atemperature below the Tg of the film material, and then the secondsurface could be heated to a temperature at least equal to the Tg of thefilm material to permit a structure to be imparted to the second surfaceof the film. However, this approach has similar drawbacks (increasedcosts required to heat the film twice, high likelihood of changes in thefirst structure when the film is reheated to impart the secondstructure, and the like). In particular, the pressure necessary to formthe structure on the second surface in such a sequential process mayresult in damage to the structure on the first surface.

One particularly desirable method for forming structured surfaces onboth major surfaces of an article is to simultaneously contact bothmajor surfaces of the article to structuring tools while the article isin the molten and flowable state. In this way, both structured surfacesare formed in a single process.

However, when the structured surface is to include protrusions, thisprocess can become complicated. In order to form protrusions on thestructured surface, the structuring tool contains depressions. Theflowable material, typically molten material, of the article flows intothe depressions of the structuring tool to form the desired surfaceprotrusions. When structures are formed on both major surfaces of thearticle simultaneously, a large mass of molten material has to flow intwo different directions. Depending upon the thickness of the articlebeing structured, when there is significant overlap of patterns, themolten material may not have sufficient volume to flow into bothdepressions on the structuring tools. This lack of filling of thedepression in the tool results in protrusions that are incomplete, thatis to say, protrusions that are not as high or wide as they are intendedto be. Articles with incomplete protrusions are generally unacceptable.

Another issue with simultaneously imparting identical structures on bothsides of a flowable material composition, especially a molten materialcomposition, relates to removing the tools from the molten materialcomposition after patterning. This is particularly problematic when themolten material composition is passed between two structured tool rolls.It has been observed when a single side of a molten material compositionis structured, that is to say when the molten material composition ispassed between a structured tool roll and a flat tool roll, that themolten material composition, which is cooling during the patterning,prefers to stay in contact with the patterned roll. This isunderstandable since, because of the patterning, the area of contactbetween the patterned roll and the cooling molten material is greaterthan the area of contact between the flat roll and the cooling moltenmaterial, and therefore the energy required for the cooling moltenmaterial to pull away from the patterned roll is greater than the energyrequired for the cooling molten material to pull away from the flatroll. However, when identical structures are formed on both sides of themolten material composition, the molten material composition does nothave a preference for one tool or the other. In some instances, this canlead to the molten material composition staying in contact with bothtools and this can cause defects in the patterned surface such asridge-like structures formed on the patterned surface.

Another complicating issue resulting from forming film articles thathave exactly matching patterns on both surfaces, is the film toughnessof such films. When the patterns are matched on both surfaces of theformed film, the overlapping protrusions form a weak point in the film.In downstream processes, such as, for example, slitting of the film, thepresence of these weak points enables edge fracture to propagate easilythrough the structured film article.

These difficulties are observed not only when the structuring is done ina continuous process, such as passing a web of material between twostructuring tools, but also in batch processes, such as stamping typeoperations. This effect is particularly present in the formation ofthinner articles. However, it is thinner articles that are desirable inmany instances.

It might seem that a simple remedy to the difficulties described aboveis to offset the tools to form patterns that are not aligned in the downweb direction. In other words, both the patterns would be perfectlyparallel when viewed in the down web and cross web directions, but whena cross section is viewed in the cross web direction, the protrusionsare not aligned. However, such a process is far from simple in practice.For example, when the process is carried out in a continuous fashionwith the two patterns imparted by passing the a flowable material, suchas a molten material, between two tool rolls, it quickly becomesapparent that such a process is in fact very difficult to implement.With such a scheme, even very small changes in processing conditions cancause the patterns to again overlap, generating all of the difficultiesdescribed above. For example, overlapping regions of repeating patternscan be caused by very small diameter differences in the tool rolls. Atconstant speed, the two patterns will drift in and out of phase(overlap) due to the diameter mismatch. To alleviate this, sophisticatedcontrol systems must be employed to prevent pattern overlap. Overlappingpatterns can also result from lateral movement of the two rolls relativeto each other, again requiring precision design controls to minimizelateral movement in one or both tool rolls. As the size of the patternsdecrease, the ability to control relative alignment becomes more andmore challenging.

Disclosed herein are articles that are structured on both major surfacesand methods for preparing these articles. The patterns on the differentsurfaces are offset at an angle, meaning that they are not perfectlyaligned. Intentionally misaligning the tool patterns to minimize theoverlap of structures reduces the need for stringent process control.The offset patterns are described in greater detail below.

The result of the angled offset patterning methods of this disclosure isthat the nature of the patterning, namely that depressions in the twostructuring tools do not need to be filled simultaneously at all pointsin the structure, permits the formation of complete protrusions in bothsurfaces, and reduces or eliminates the presence of incompleteprotrusions. Additionally, the angled offset patterning assists ineliminating defects caused by the flowable material composition stickingto both tools, since the angled offset causes the flowable materialcomposition to prefer one tool over the other tool. Also, since themajority of protrusions on the two structured surfaces are notoverlapping, the film toughness issue is also minimized or eliminated.

FIGS. 1, 2 and 3 illustrate the two processes described above. FIG. 1shows an overview of a continuous process to form a film article bysimultaneously structuring both sides of a flowable materialcomposition, in particular a molten material composition. FIGS. 2 and 3show a view of the portion of the process where the two tool structurescontact the flowable material composition. FIG. 2 shows a comparativeprocess for forming double sided patterns that are not offset. FIG. 3shows an embodiment of the process of this disclosure.

In FIG. 1, 110 is an extruder or similar device for supplying a flowablematerial composition 120. The flowable material composition 120 iscarried on first structured tool roll 115 to the point where theflowable material composition contacts second structured tool roll 105,causing the simultaneous microstructuring of flowable materialcomposition 120. Non-structured roll 125 is an optional take-up rollthat aids in removing the dual structured film from structured roll 115.In FIG. 2, which illustrates microstructuring process 100, flowablematerial composition 120 passes between two microstructuring tools, Tool1 is 130 and corresponds to tool roll 105 in FIG. 1, and Tool 2 is 140and corresponds to tool roll 115 in FIG. 1. The flowable materialcomposition 120 does not completely fill the depressions in the Tools130 and 140, rendering a region of incomplete fill, 150. Upon removal ofthe Tools 130 and 140, the formed dual-structured article comprisesincomplete protrusions 160 with land 170 between the protrusions. Thethickness between the base of the protrusions and the second majorsurface in microstructured surfaces is often described as the “land”.The height of protrusions is often measured relative to the top surfaceof the land. While the figure is not drawn to scale, the land betweenthe protrusions 170 is thinner than the flowable material composition120, since some of the flowable material composition 120 has been formedinto the protrusions 160.

In FIG. 3, Process 200 of this disclosure is illustrated. In thisprocess, flowable material composition 120 is simultaneously contactedby two microstructuring tools, Tool 1 is 230 and corresponds to toolroll 105 in FIG. 1, and Tool 2 is 240 and corresponds to tool roll 115in FIG. 1. The flowable material composition 120 completely fills thedepressions in the Tools 230 and 240, filling the region 250, which inFIG. 2 was incompletely filled. Upon removal of the Tools 230 and 240,the formed dual-structured article comprises complete protrusions 260with land 270 between the protrusions. While the figure is not drawn toscale, the land between the protrusions 270 is thinner than the flowablematerial composition 120, since some of the flowable materialcomposition 120 has been formed into the protrusions 260. Also, itshould be noted that if the flowable material compositions 120 in FIGS.2 and 3 are the same thickness, the land 270 of FIG. 3 is expected to bethinner than the land 170 of FIG. 2 because protrusions 260 of FIG. 3are larger (and therefore contain more mass from the flowable materialcomposition 120) than protrusions 160 of FIG. 2. It should also be notedthat FIGS. 2 and 3 are generalized to show any type of patterning tools,and if the tools in FIGS. 2 and 3 are tool rolls such as are shown inFIG. 1, the structured film comprising structures 160 or 260 and land170 or 270 would remain in contact with one of the tool rolls afterstructuring.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein. The recitation of numerical ranges byendpoints includes all numbers subsumed within that range (e.g. 1 to 5includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within thatrange.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” encompass embodiments having pluralreferents, unless the content clearly dictates otherwise. For example,reference to “a layer” encompasses embodiments having one, two or morelayers. As used in this specification and the appended claims, the term“or” is generally employed in its sense including “and/or” unless thecontent clearly dictates otherwise.

As used herein, the term “polymer” refers to a polymeric material thatis a homopolymer or a copolymer. As used herein, the term “homopolymer”refers to a polymeric material that is the reaction product of onemonomer. As used herein, the term “copolymer” refers to a polymericmaterial that is the reaction product of at least two differentmonomers.

As used herein, the term “ordered arrangement” when used to describemicrostructural features, especially a plurality of microstructures,means an imparted pattern different from natural surface roughness orother natural features, where the arrangement can be continuous ordiscontinuous, can be a repeating pattern, a non-repeating pattern, etc.

As used herein, the term “microstructure” means the configuration offeatures wherein at least 2 dimensions of the features are microscopic.The topical and/or cross sectional view of the features must bemicroscopic.

As used herein, the term “microscopic” refers to features of smallenough dimension so as to require an optic aid to the naked eye whenviewed from any plane of view to determine its shape. One criterion isfound in Modern Optic Engineering by W. J. Smith, McGraw-Hill, 1966,pages 104-105 whereby visual acuity, “ . . . is defined and measured interms of the angular size of the smallest character that can berecognized.” Normal visual acuity is considered to be when the smallestrecognizable letter subtends an angular height of 5 minutes of arc onthe retina. At a typical working distance of 250 mm (10 inches), thisyields a lateral dimension of 0.36 mm (0.0145 inch) for this object.

The term “adjacent” as used herein when referring to two layers meansthat the two layers are in proximity with one another with nointervening open space between them. They may be in direct contact withone another (e.g. laminated together) or there may be interveninglayers.

As used herein, the term “unitary” when used to describe a substrate orfilm means that the substrate or film that has been prepared in singlestep. The unitary substrates or films may be prepared from a singlematerial or multiple materials. Unitary substrates or films may beprepared as a monolithic substrate or film (such as extrusion, forexample) or it may be prepared as a multi-layer substrate or film (suchas by co-extrusion, for example).

As used herein the term “thermoplastic”, refers to the property of apolymeric material. Thermoplastic materials are materials which meltand/or flow upon the application of heat, resolidify upon cooling andagain melt and/or flow upon the application of heat. The thermoplasticmaterial undergoes only a physical change upon heating and cooling, noappreciable chemical change occurs.

As used herein, the term “major axis” refers to an axis of translationalsymmetry present in a pattern of regular shapes. A pattern of shapesexhibits translational symmetry if displacement in adirection-horizontal or vertical displacement for example—returns theshape to essentially its original configuration. For instance, in asquare pattern there are two major axes of translational symmetry, onein the x direction and one in the y direction. In a hexagonally packedarray, there are three major axes of translation symmetry.

Disclosed herein are methods for preparing articles with twomicrostructured surfaces. These methods comprise providing a flowablematerial composition comprising a first major surface and second majorsurface, providing a first microstructuring tool, the firstmicrostructuring tool comprising a structured surface comprising apattern comprising a plurality of depressions, providing a secondmicrostructuring tool, the second microstructuring tool comprising astructured surface comprising a pattern comprising a plurality ofdepressions, and simultaneously contacting the first microstructuringtool to the first major surface of the flowable material composition andcontacting the second microstructuring tool to the second major surfaceof the flowable material composition to form structured first and secondmajor surfaces on the flowable material composition. While, dependingupon the configurations of the two structuring tools, the twostructuring tools may not contact the flowable composition at exactlythe same instant, it is typical that they contact the flowablecomposition at substantially the same time, and the term“simultaneously” is meant to convey this concept and also that the twotools are in contact with the flowable composition at the same time.

The structured first and second major surfaces are formed such that eachcomprise a plurality of spaced apart protrusions forming a repeatingpattern, each repeating pattern having a major axis, wherein the majoraxis comprises one of the major axes in the translational direction ofthe repeating pattern, and wherein the major axis of the repeatingpattern on the second major surface forms an oblique angle with themajor axis on the first major surface, wherein the angle is in the rangeof 10-90% of the angle of rotational symmetry of the repeating pattern.In some embodiments, the angle is the range of 20-80% of the angle ofrotational symmetry of the repeating pattern. In some embodiments, therepeating pattern comprises a repeating square pattern and the anglebetween the major axis on the repeating pattern on the first majorsurface and the major axis on the repeating pattern on the second majorsurface is in the range of 20-70°.

Typically, the flowable material composition comprises a unitary film,sometimes called a unitary substrate. A unitary film is one which maycomprise a single material or it may comprise multiple materials, butthe unitary film is one that is formed in a single step. In someembodiments, the unitary film comprises a monolithic construction. Bythis it is meant that the unitary film is formed of a single material,frequently by a process such as extrusion. In these embodiments, theflowable material composition typically is the output of an extruder. Inother embodiments, the unitary film may comprise a blend of materials.In other embodiments, the unitary film comprises a multi-layerconstruction, even though the unitary film is prepared in a single step.Thus, formation of a multi-layer film or substrate by lamination ofindividual film layers typically does not provide a unitary film orsubstrate. In embodiments of this disclosure, the flowable materialcomposition typically is the output of extrusion or co-extrusion and theextruded or co-extruded layers may comprise a single material or may bea blend of materials.

A wide range of materials are suitable for use as the flowable materialcomposition. Typically these materials are thermoplastic polymericmaterials. Examples of useful thermoplastic polymeric materials includepolyvinyl chloride, polysulfones, polyalkylenes such as polyethylene,polypropylene and polybutylene, polyesters such as PET (polyethyleneterephthalate), copolyesters such as PETG, polycarbonates,poly(meth)acrylates such as PMMA (polymethyl methacrylate), nylons, TPOs(thermoplastic polyolefin blends), polyurethanes including TPU(thermoplastic polyurethane materials), polystyrenes, impact-modifiedpolystyrenes, and the like. Particularly suitable materials includepolypropylene, polystyrenes, and impact-modified polystyrenes.

As mentioned above, the flowable material composition is, in manyembodiments, a unitary film. This unitary film can be prepared byextrusion, either extrusion of a single layer or co-extrusion ofmultiple layers. Typically this unitary film has a thickness of 25-203micrometers (1-8 mils). When the thickness of the unitary film includesthe height of the structures the thickness can be even larger, from25-1,016 micrometers (1-40 mils).

Both major surfaces of the flowable material composition aresimultaneously contacted by microstructuring tools. Microstructuringtools are well understood by one of skill in the art to impart astructure to a surface when contacted to a structurable surface underthe conditions of heat and pressure. In this disclosure, the majorsurfaces of the flowable material composition are structurable surfaces,and upon release of the microstructuring tool, the surfaces of theflowable material composition are structured surfaces. The structure onthe structured surface is the inverse of the structure on the toolsurface, that is to say a protrusion on the tool surface will form adepression on the structured surface, and a depression on the toolsurface will form a protrusion on the structured surface. Themicrostructural features may assume a variety of shapes, but at leastsome of the microstructural features on the microstructuring tools aredepressions.

Typically, the microstructuring tool is a molding tool. Structuredmolding tools can be in the form of a planar stamping press, a flexibleor inflexible belt, or a roller. Furthermore, molding tools aregenerally considered to be tools from which the microstructured patternis generated in the surface by embossing, coating, casting, or platenpressing and do not become part of the finished article.

A broad range of methods are known to those skilled in this art forgenerating microstructured molding tools. Examples of these methodsinclude but are not limited to photolithography, etching, dischargemachining, ion milling, micromachining, and electroforming.Microstructured molding tools can also be prepared by replicatingvarious microstructured surfaces, including irregular shapes andpatterns, with a moldable material such as those selected from the groupconsisting of crosslinkable liquid silicone rubber, radiation curableurethanes, etc. or replicating various microstructures by electroformingto generate a negative or positive replica intermediate or finalembossing tool mold. Also, microstructured molds having random andirregular shapes and patterns can be generated by chemical etching,sandblasting, shot peening or sinking discrete structured particles in amoldable material. Additionally any of the microstructured molding toolscan be altered or modified according to the procedure taught in U.S.Pat. No. 5,122,902 (Benson). The tools may be prepared from a wide rangeof materials including metals such as nickel, copper, steel, or metalalloys, or polymeric materials.

A variety of patterns may be present in the structured surface of themicrostructuring tools. In articles with dual sided arrays of structuressuch as are generated by the methods described in this disclosure, it isoften desirable for the patterns of the structures to be the same. Inthis way, both sides of the film can be used for the same purpose.However, as described above, it can be undesirable to form identicalprotrusion structures in both major surfaces simultaneously, because theflowable material composition may not be able to completely form bothsets of protrusions simultaneously, and this can lead to the formationof incomplete structural features. In this disclosure, the patterns ofstructures are angularly offset, meaning that they are not perfectlyaligned. In this way, even if the patterns of structures have the samesizes and shapes, because they are offset, the problems with formingsets of protrusions over large contiguous portions of the surface areaon both major surfaces of the flowable material compositionsimultaneously are avoided. In the case of tools that are rolls forminga nip, as the tool rolls change relative position during processing, thein phase and out of phase overlap is minimized.

There are several ways in which the offset patterns can be described. Ingeneral, the patterns can be described by the major axes of thepatterns. In this description, the structured first and second majorsurfaces comprise a plurality of spaced apart protrusions forming arepeating pattern, each repeating pattern having a major axis, whereinthe major axis comprises one of the major axes in the translationaldirection of the repeating pattern, and wherein the major axis of therepeating pattern on the second major surface forms an oblique anglewith the major axis on the first major surface, wherein the angle is inthe range of 10-90% of the angle of rotational symmetry of the repeatingpattern. In some embodiments, the angle is the range of 20-80% of theangle of rotational symmetry of the repeating pattern. An example ofsuch a pattern is a repeating square pattern. In this example, the angleof rotational symmetry is 90°, and the offset angle is thus in the rangeof 9-81°, or 18-72°. The particular angle chosen depends not only on thespecific design of the pattern, but also on a variety of other factors.For example, in the square pattern designs shown in FIG. 6, the widthand depth of each protrusion that forms the pattern are also factors tobe considered when the offset angle is established. In some embodiments,the repeating pattern comprises a repeating square pattern and the anglebetween the major axis on the repeating pattern on the first majorsurface and the major axis on the repeating pattern on the second majorsurface is in the range of 20-70°.

Another way to describe the offset pattern, one that can be used todescribe many embodiments, is one in which the protrusions formed in themajor surfaces of the flowable material composition form ridges. Forexample, where each of the first and second structured surfacescomprises a plurality of parallel spaced apart ridges extending along afirst direction intersecting a plurality of parallel spaced apart ridgesextending along a second direction perpendicular to the first directionto form an array of cavities, each cavity being defined by four walls,the walls of the cavities on the first and second structured surfaceshaving a same height and width. The first direction in the firststructured surface forms an oblique angle in the range of 20° to 70°with the first direction of the second structured surface.

Yet another way of describing the offset patterns is to consider a crosssection of the formed unitary film article, particularly a cross sectiontaken along the first direction as described above. The cross sectioncomprises a plurality of discrete spaced apart structures on the firstmajor surface (first structures) and a plurality of discrete spacedapart structures in the second major surface (second structures). Atleast one of the first structures fully overlaps a second structure, andat least one first structure does not fully overlap a second structure.This is in contrast to the situation where the structures were notoffset, where all of the top and bottom structures would completelyoverlap.

In some embodiments, it may be desirable that the protrusions formed inthe first and second structured surfaces are not ridges. In theseembodiments, the offset patterns may be described as comprising opposingfirst and second major surfaces, each of the first and second majorsurfaces comprising a regular two-dimensional array of substantiallyidentical discrete spaced apart protruding structures forming rows ofprotruding structures extending along a first direction and columns ofprotruding structures extending along a second direction perpendicularto the first direction, the protruding structures in the top and bottomsurfaces being substantially identical. In these embodiments, the firstdirection in the first structured surface forms an oblique angle in therange of 20° to 70° with the first direction of the second structuredsurface.

Typically, the protrusions of the structured first and second surfacesare arranged to form an array of cavities. In some embodiments, thearray of cavities comprises a square array of cavities. The cavities canalso comprise a variety of other shapes, such as hexagons, triangles,and circles. The cavities can be described as having walls formed by theprotrusions and bottoms formed by the land area between the formedprotrusions. (The land that forms the cavity bottoms has been describedabove relative to FIGS. 2 and 3.) One advantage of the methods of thepresent disclosure is that the land between protrusions can berelatively thin. Typically, this thickness is 5-200 micrometers, 5-100micrometers, or even 10-50 micrometers. It is desirable that the landbetween protrusions be relatively thin because it permits the entirearticle to be relatively thin and this permits the dual sided structuralarticles to be prepared from less material and thus be less expensive.Additionally, having thin land thicknesses can help the optical clarity,which is desirable if the dual-sided structured films are used, forexample, in DNA microtest wells, and can aid in the processing of thefilms, such as, for example, the making of through-film perforations bypost-structuring flame treating to form filtration media.

Another way to describe the relative thinness of the cavity bottoms isillustrated in FIG. 4. In this figure, a cross section of an article ofthis disclosure is shown. In this cross section, protrusions 160 as wellas land 170 are shown. Protrusions 160 and the cavity that it definesare shown as Area A and Area B. This combined surface area is comparedto the area of the land which is Area C. Thus the ratio of the sums ofAreas A and B to the Area C is greater than 1:1. In some embodiments theratio of the sums of Areas A and B to the Area C is in the range of 1:1to 20:1.

Typically the heights of the protrusions on the first and secondstructured major surfaces are typically much greater than the landthickness. Additionally, when the structured substrate is viewed in across section, the cross section of a ridge will have a cross sectionalarea (a first cross sectional area). When this first cross sectionalarea is compared to the cross sectional area of the land beneath theridge, the ratio of the first cross sectional area to the land crosssectional area is at least one.

Also disclosed herein are articles with microstructures on both majorsurfaces, called structured substrates. These articles are unitaryarticles and can be formed by the methods described above. As describedabove, the microstructured patterns on the first major surface is offsetfrom the microstructured pattern of the second major surface.

In some embodiments, the article comprises a unitary substrate withopposing first and second major surfaces, each of the first and secondmajor surfaces comprising a plurality of parallel spaced apartprotrusions forming a repeating pattern, each repeating pattern having amajor axis, wherein the major axis comprises one of the major axes inthe translational direction of the repeating pattern, and wherein themajor axis of the repeating pattern on the second major surface forms anoblique angle with the major axis on the first major surface, whereinthe angle is in the range of 10-90% of the angle of rotational symmetryof the repeating pattern, and wherein the structured substrate is aunitary substrate. In some embodiments, the angle is the range of 20-80%of the angle of rotational symmetry of the repeating pattern.

In some embodiments, the repeating pattern on the first major surfaceand/or the second major surface comprise a periodic geometric pattern. Awide variety of periodic geometric patterns are suitable. Examples ofsuitable periodic geometric patterns include a pattern of squares, apattern of hexagons, a pattern of triangles, or a pattern of circles.These geometric patterns form an array of cavities in the structuredsurface. Therefore, such arrays include arrays of square cavities,hexagonal cavities, triangular cavities, or circular cavities. In someembodiments, the repeating pattern comprises a repeating square patternand the angle between the major axis on the repeating pattern on thefirst major surface and the major axis on the repeating pattern on thesecond major surface is in the range of 20-70°.

The cavities can be described as having walls formed by the protrusionsand bottoms formed by the land area between the formed protrusions. (Theland that forms the cavity bottoms has been described above relative toFIGS. 2 and 3.) One advantage of the methods of the present disclosureis that the land between protrusions can be relatively thin. Typically,this thickness is 5-200 micrometers, 5-100 micrometers, or even 10-50micrometers. It is desirable that the land between protrusions berelatively thin because it permits the entire article to be relativelythin and this permits the dual sided structural articles to be preparedfrom less material and thus be less expensive. Additionally, having thinland thicknesses can help the optical clarity, which is desirable if thedual-sided structured films are used, for example, in DNA microtestwells, and can aid in the processing of the films, such as, for example,the making of through-film perforations by post-structuring flametreating to form filtration media.

Another way to describe the relative thinness of the cavity bottoms isillustrated in FIG. 4. In this figure, a cross section of an article ofthis disclosure is shown. In this cross section, protrusions 160 as wellas land 170 are shown. Protrusions 160 and the cavity that it definesare shown as Area A and Area B. This combined surface area is comparedto the area of the land which is Area C. Thus the ratio of the sums ofAreas A and B to the Area C is greater than 1:1. In some embodiments theratio of the sums of Areas A and B to the Area C is in the range of 1:1to 20:1.

Typically the heights of the protrusions on the first and secondstructured major surfaces are typically much greater than the landthickness. Additionally, when the structured substrate is viewed in across section, the cross section of a ridge will have a cross sectionalarea (a first cross sectional area). When this first cross sectionalarea is compared to the cross sectional area of the land beneath theridge, the ratio of the first cross sectional area to the land crosssectional area is at least one.

The offset of the patterns of the first and second structured surfacesis an angular offset, not a lateral or longitudinal offset. This isshown in FIGS. 5 and 6. FIG. 5 is a comparative dual structuredsubstrate. In FIG. 5 the solid lines describe the pattern in the firstmajor surface and the dashed lines describe the pattern in the secondmajor surface. The major axis for the solid line pattern is shown as510, and the major axis for the dashed line pattern is shown as 520. Thepatterns are offset but only in the lateral direction. It is clear thatthere is no angular offset because the angular offset angle as describedby 510 and 520 is 0°. These patterns can be described as angularlyaligned instead of angularly offset as are the patterns of thisdisclosure.

FIG. 6 shows a dual structured substrate of this disclosure. In FIG. 6the solid lines describe the pattern in the first major surface and thedashed lines describe the pattern in the second major surface. The majoraxis for the solid line pattern is shown as 610, and the major axis forthe dashed line pattern is shown as 620. The patterns are angularlyoffset because the angular offset angle as described by 610 and 620 isgreater than 0° but less than 90°.

EXAMPLES

A series of computer modeling studies were carried out to model arraysof regular structures at differing angular offsets for dual-sidedmicrostructured substrates.

Example 1 Modeling of a Square Array Pattern

The computer modeling software package Blender (available fromhttp://www.blender.org) was used to create a two dimensionalrepresentation of the protrusions on the substrate.

The pattern on one side of the substrate was represented by a mesh of64×64 close-packed array of squares each with an outside side length of1 Unit and a side thickness or width, W, of 0.05 Units. This tiled arraywith a repeat distance or pitch, P, of 1 Unit, when viewed perpendicularto the plane of the array looks like an array of horizontal and verticalrectangles. The pattern on the second side of the substrate was anidentical array duplicated from the first. This second mesh object wassuperimposed on top of the first array with zero offset in eithertranslation or rotation.

Both the arrays were given a color, in this case red (RGB 255,0,0) and atransparency (alpha) value of 50%, such that when rendered using theinternal Blender Renderer with suitable settings there was aquantifiable difference in the brightness value of the image betweenareas where there was and was not overlap of the two patterns.

A virtual orthogonal camera was positioned perpendicular to the plane ofthe rectangles such that the rendered image covered an area with a widthand height equal to 40 Units. The image was centered on the center ofthe intersection the edges connecting four neighboring squares at thecenter of one of the arrays. The rendered image was saved in theMicrosoft bitmap format, 24 bits per pixel, 72 dpi and at a size of1600×1600 pixels.

To evaluate the proportion of overlap of the two arrays, one of thearrays was translated in a grid-like fashion in steps of 0.05 Units inboth the X- and Y-directions to a maximum offset of 0.95 Units in boththe X- and Y-directions to cover one “unit cell” area, i.e. 400 uniquepositions. For each offset position a rendered image was saved asdescribed above and then opened in the image analysis software packageImageJ (available from http://rsbweb.nih.gov/ij/). The Histogramfunction was used to generate 256 level histogram data from the image.For the conditions used above, the overlapped and non-overlapped areasof the image had values of 75 and 63 respectively. This tabulatedhistogram data was recorded.

After all 400 positions had been recorded, the translation offset wasreset to zero and the rotation of one array modified and the generatingand recording of the data for each of the 400 translation offsetpositions was repeated. This process was completed for rotation offsetsof 0, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, and 45 degrees. Thecenter of rotation was located at the center of the intersection theedges connecting four neighboring squares.

Table 1 below shows the results of the analysis. The maximum possibleoverlap occurred when the patterns are fully aligned at zero degreesrotational offset and zero positional offset. The first column shows thedegrees of angular offset of one pattern relative to the other. Thesecond column shows the average overlap of all possible translationaloffset positions in proportion to the maximum possible overlap. Thesecond and third columns show the smallest and largest value of overlapover all possible translational offset positions in proportion to themaximum possible overlap. The fifth column shows the difference ofcolumns three and four. Values for angle offsets of between 45 and 90°are inferred by symmetry.

TABLE 1 Average % Minimum % Maximum % Offset of maximum of maximum ofmaximum Angle possible possible possible Range of (Degrees) overlapoverlap overlap Overlap % 0 38.7% 22.2% 100.0% 77.8% 1 36.3% 23.4% 42.3%18.9% 2 35.8% 32.0% 42.2% 10.2% 3 36.1% 35.3% 38.2% 2.9% 4 36.0% 33.3%37.1% 3.8% 5 35.9% 34.0% 38.1% 4.1% 10 36.0% 35.9% 36.1% 0.2% 15 36.0%35.9% 36.2% 0.3% 20 36.0% 35.9% 36.1% 0.1% 25 36.0% 36.0% 36.0% 0.1% 3036.0% 35.9% 36.1% 0.3% 35 36.0% 36.0% 36.0% 0.0% 40 36.0% 36.0% 36.0%0.0% 45 35.9% 35.9% 35.9% 0.0% 50 36.0% 36.0% 36.0% 0.0% 55 36.0% 36.0%36.0% 0.0% 60 36.0% 35.9% 36.1% 0.3% 65 36.0% 36.0% 36.0% 0.1% 70 36.0%35.9% 36.1% 0.1% 75 36.0% 35.9% 36.2% 0.3% 80 36.0% 35.9% 36.1% 0.2% 8535.9% 34.0% 38.1% 4.1% 86 36.0% 33.3% 37.1% 3.8% 87 36.1% 35.3% 38.2%2.9% 88 35.8% 32.0% 42.2% 10.2% 89 36.3% 23.4% 42.3% 18.9% 90 38.7%22.2% 100.0% 77.8%

Example 2 Modeling of a Triangular Array Pattern

The same procedures as Example 1 were followed except for the followingchanges. The two identical arrays consisted of triangles with an outsideedge length of 1 Unit, an edge width, W, of 0.028875 Units in aclose-packed array with one of the edges of the triangles in theX-direction. In this case one of the arrays was translated by 0.05 Unitsin the X-direction to a maximum of 1.5 Units, and 0.050943 Units in theY-Direction to a maximum of 0.815088 Units.

Rotation angles of 0, 1, 2, 3, 4, 5, 10, 15, 20, 25 and 30 degrees wereevaluated. The center of rotation was located at the center of theintersection the edges connecting six neighboring triangles.

The data are presented in Table 2 below, in the same manner as for thedata described in Example 1. Values for angle offsets of between 30 and60° are inferred by symmetry.

TABLE 2 Average % Minimum % Maximum % Offset of maximum of maximum ofmaximum Angle possible possible possible Range of (Degrees) overlapoverlap overlap Overlap % 0 37.2% 28.1% 100.0% 71.9% 1 36.0% 33.3% 38.5%5.2% 2 36.0% 34.6% 36.9% 2.4% 3 36.0% 35.5% 36.8% 1.2% 4 36.0% 35.7%36.6% 1.0% 5 36.0% 35.9% 36.1% 0.2% 10 36.0% 36.0% 36.1% 0.1% 15 35.9%35.3% 36.7% 1.5% 20 36.0% 36.0% 36.0% 0.1% 25 36.0% 36.0% 36.0% 0.1% 3036.0% 35.8% 36.2% 0.4% 35 36.0% 36.0% 36.0% 0.1% 40 36.0% 36.0% 36.0%0.1% 45 35.9% 35.3% 36.7% 1.5% 50 36.0% 36.0% 36.1% 0.1% 55 36.0% 35.9%36.1% 0.2% 56 36.0% 35.7% 36.6% 1.0% 57 36.0% 35.5% 36.8% 1.2% 58 36.0%34.6% 36.9% 2.4% 59 36.0% 33.3% 38.5% 5.2% 60 37.2% 28.1% 100.0% 71.9%

Example 3 Modeling of a Hexagonal Array Pattern

The same procedures as Example 1 were followed except for the followingchanges. The two identical arrays consisted of hexagons with thesmallest diameter of the outside edge of the Hexagon of 1 Unit, an edgewidth, W, of 0.05 Units, and a in a hexagonal close-packed array withthe orientation of the smallest diameter in the X-direction. In thiscase one of the arrays was translated by 0.05 Units in the X-directionto a maximum of 1.5 Units, and 0.050943 Units in the Y-Direction to amaximum of 0.815088 Units. This defined a unit cell of the pattern.

Rotation angles of 0, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35,40, 45, 50,55 and 60 degrees were evaluated. The center of rotation was located atthe center of the intersection of the edges connecting three neighboringhexagons.

The data are presented in Table 3 below, in the same manner as for thedata described in Example 1. Values for angle offsets of between 30 and60° are inferred by symmetry.

TABLE 3 Average % Minimum % Maximum % of maximum of maximum of maximumOffset Angle possible possible possible Range of (Degrees) overlapoverlap overlap Overlap % 0 36.8% 22.2% 100.0% 77.8% 1 36.0% 31.6% 42.2%10.6% 2 35.9% 33.0% 38.0% 5.0% 3 36.0% 34.7% 37.7% 2.9% 4 36.0% 35.1%36.9% 1.7% 5 36.0% 35.8% 36.1% 0.4% 10 36.0% 35.9% 36.1% 0.2% 15 36.0%35.9% 36.0% 0.1% 20 36.0% 35.9% 36.1% 0.1% 25 36.0% 35.9% 36.1% 0.2% 3036.0% 35.8% 36.3% 0.4% 35 36.0% 35.9% 36.1% 0.2% 40 36.0% 35.9% 36.1%0.1% 45 36.0% 35.9% 36.0% 0.1% 50 36.0% 35.9% 36.1% 0.2% 55 36.0% 35.8%36.1% 0.4% 56 36.0% 35.1% 36.9% 1.7% 57 36.0% 34.7% 37.7% 2.9% 58 35.9%33.0% 38.0% 5.0% 59 36.0% 31.6% 42.2% 10.6% 60 36.8% 22.2% 100.0% 77.8%

What is claimed is:
 1. A structured substrate comprising: a first majorsurface and second major surface, wherein the first major surface andthe second major surface each comprise a plurality of spaced apartprotrusions forming a repeating pattern, each repeating pattern having amajor axis, wherein the major axis comprises one of the major axes inthe translational direction of the repeating pattern, and wherein themajor axis of the repeating pattern on the second major surface forms anoblique angle with the major axis on the first major surface, whereinthe angle is in the range of 10-90% of the angle of rotational symmetryof the repeating pattern, and wherein the structured substrate is aunitary substrate.
 2. The structured substrate of claim 1, wherein therepeating pattern on the first major surface and/or the second majorsurface comprise any periodic geometric pattern.
 3. The structuredsubstrate of claim 2, wherein the periodic geometric pattern comprises apattern of squares, a pattern of hexagons, a pattern of triangles, or apattern of circles.
 4. The structured substrate of claim 1, wherein therepeating pattern on the first major surface is the same as therepeating pattern on the second major surface.
 5. The structuredsubstrate of claim 1, wherein the repeating pattern on the first majorsurface is different from the repeating pattern on the second majorsurface.
 6. The structured substrate of claim 1, wherein the angle is inthe range of 20-80% of the angle of rotational symmetry of the repeatingpattern.
 7. The structured substrate of claim 1, wherein the repeatingpattern on the first major surface and the repeating pattern on thesecond major surface form an array of cavities, and wherein theprotrusions form the walls of the cavities.
 8. The structured substrateof claim 7, wherein the array of cavities comprises an array of squarecavities.
 9. The structured substrate of claim 7, wherein the array ofcavities comprises an array of hexagonal cavities.
 10. The structuredsubstrate of claim 7, wherein the array of cavities comprises an arrayof triangular cavities.
 11. The structured substrate of claim 7, whereinthe array of cavities comprises an array of circular cavities.
 12. Thestructured substrate of claim 7, wherein in a cross sectional view alongthe first major axis, a first surface area is defined as the region ofthe first major surface comprising a protrusion and the cavity for whichthe protrusion forms one wall, a second surface area is defined as theregion of the second major surface comprising a protrusion and thecavity for which the protrusion forms one wall, and a third surface areais defined as the land area between the first surface area and thesecond surface area, and wherein the ratio of the sum of the firstsurface area and the second surface area to the third surface area is1:1 or greater.
 13. The structured substrate of claim 1 being made byextrusion replication.
 14. The structured substrate of claim 1, whereinthe structured substrate comprises a uniform material composition.
 15. Amethod of preparing an article comprising: providing a flowable materialcomposition comprising a first major surface and second major surface;providing a first microstructuring tool, the first microstructuring toolcomprising a structured surface comprising a pattern comprising aplurality of depressions; providing a second microstructuring tool, thesecond microstructuring tool comprising a structured surface comprisinga pattern comprising a plurality of depressions; and simultaneouslycontacting the first microstructuring tool to the first major surface ofthe flowable material composition and contacting the secondmicrostructuring tool to the second major surface of the flowablematerial composition to form structured first and second major surfaceson the flowable material composition, wherein the structured first majorsurface and the structured second major surface each comprise aplurality of spaced apart protrusions forming a repeating pattern, eachrepeating pattern having a major axis, wherein the major axis comprisesone of the major axes in the translational direction of the repeatingpattern, and wherein the major axis of the repeating pattern on thesecond major surface forms an oblique angle with the major axis on thefirst major surface, wherein the angle is in the range of 10-90% of theangle of rotational symmetry of the repeating pattern.
 16. The method ofclaim 15, wherein the flowable material composition comprises a unitaryfilm.
 17. The method of claim 16, wherein unitary film comprises amonolithic construction.
 18. The method of claim 16, wherein the unitaryfilm comprises a multi-layer construction.
 19. The method of claim 15,wherein providing the flowable material composition comprises extrusionof a unitary film.
 20. The method of claim 19, wherein extrusioncomprises co-extrusion.
 21. The method of claim 16, wherein the unitaryfilm, prior to structuring, has a thickness of from 25-203 micrometers.22. The method of claim 15, wherein each of the first and secondstructured surfaces comprises a plurality of parallel spaced apartridges extending along a first direction intersecting a plurality ofparallel spaced apart ridges extending along a second directionperpendicular to the first direction to form an array of cavities, eachcavity being defined by four walls, the walls of the cavities on thefirst and second structured surfaces having a same height and width, thefirst direction in the first major surface forming an oblique angle in arange from 20° to 70° with the first direction in the second majorsurface.
 23. The method of claim 22, wherein the array of cavitiescomprises a square array of cavities.
 24. The method of claim 22,wherein the cavities in the first and second major surfaces areseparated by a land having a land thickness of from 25-203 micrometers.